1. Field of the Invention
This invention relates to polycrystalline diamond cutters for use in earth-boring bits. Specifically, this invention relates to polycrystalline diamond cutters which have modified substrates to selectively modify and alter residual stress in the cutter structure.
2. Statement of the Art
Polycrystalline diamond compact cutters (hereinafter referred to as xe2x80x9cPDCxe2x80x9d cutters) are well known and widely used in drill bit technology as the cutting element of certain drill bits used in core drilling, oil and gas drilling, and the like. Polycrystalline diamond compacts generally comprise a polycrystaIline diamond (hereinafter xe2x80x9cPCDxe2x80x9d) table formed on a carbide substrate by a high temperature, high-pressure (hereinafter xe2x80x9cHTHPxe2x80x9d) sintering process. The PCD table and substrate compact may be attached to an additional or larger (i.e., longer) carbide support by, for example, a brazing process. Alternatively, the PCD table may be formed on an elongated carbide substrate in a sintering process to form the PDC cutter with an integral elongated support. The support of the PDC cutter is then brazed or otherwise attached to a drill bit in a manner which exposes the PCD table to the surface for cutting.
It is known that PDC cutters, by virtue of the materials comprising the PCD table I and the support, inherently have residual stresses existing in the compact therebetween, throughout the table and the carbide substrate, and particularly at the interface. That is, the diamond and the carbide have varying coefficients of thermal expansion, elastic moduli and bulk compressibilities such that when the PDC cutter is formed, the diamond and the carbide shrink by different amounts. As a result, the diamond table tends to be in compression while the carbide substrate and/or support tend to be in tension. Fracturing of the PDC cutter can result, often in the interface between the diamond table and the carbide, and/or the cutter may delaminate under the extreme temperatures and forces of drilling.
Various solutions have been suggested in the art for modifying the residual stresses in PDC cutters so that cutter failure is avoided. For example, it has been suggested that configuring the diamond table and/or carbide substrate in a particular way may redistribute the stress such that tension is reduced, as disclosed in U.S. Pat. No. 5,351,772 to Smith and U.S. Pat. No. 4,255,165 to Dennis. Other cutter configurations which address reduced stresses are disclosed in U.S. Pat. No. 5,049,164 to Horton, et al.; U.S. Pat. No. 5,176,720 to Martell, et al.; U.S. Pat. No. 5,304,342 to Hall, Jr., et al.; and U.S. Pat. No. 4,398,952 to Drake (in connection with the formation of roller cutters).
Recent experimental testing has shown that the residual stress state of the diamond table of a PDC cutter can be controlled by novel means not previously disclosed in the literature. That is, results have shown that a wide range of stress states, from high compression through moderate tension, can be imposed on the diamond table by selectively tailoring the carbide substrate. Thus, it would be advantageous in the art to provide PDC cutter having selectively tailored stress states, and to provide methods for producing such PDC cutters.
In accordance with the present invention, a polycrystalline diamond compact cutter having a tailored carbide substrate which favorably alters the compressive stresses in the diamond table and residual tensile stresses within the carbide substrate is provided to produce a PDC cutter with improved stress characteristics. Modification of the substrate to tailor the stress characteristics in the diamond table and substrate may be accomplished by selectively thinning the carbide substrate subsequent to HTHP processing, by selectively varying the material constituents of the substrate, by subjecting the PDC cutter to an annealing process during sintering, by subjecting the formed PDC cutter to a post-process stress relief anneal, or by a combination of those means.
The PDC cutters of the present invention are comprised of a polycrstalline diamond table, a carbide substrate on which the polycrystalline diamond table is formed (e.g., sintered) and, optionally, a carbide support of typically greater thickness than either the diamond table or the carbide support to which the substrate is connected (e.g., brazed). However, it has been discovered that a wide range of stress states, from high compression through moderate tension, can be imposed in the diamond table by selectively tailoring the carbide substrate thickness. The carbide substrate may be formed with a selected thickness by the provision of sufficient carbide material during the HTHP sintering process to produce the desired thickness. In addition, or alternatively, once the PDC cutter is formed, the substrate may be selectively thinned by subjecting it to a grinding process or machining or by electro-discharge machining processes.
It has been shown through experimental and numerical residual stress analyses that the magnitude of stress existing in the diamond table is related to the thickness of the support. Thus, within a suitable range, the carbide substrate of the cutter may be thinned to achieve a desired magnitude of stress in the diamond table appropriate to a particular use. The achievement of an appropriate or desired degree of thinness in the carbide support, and therefore the desired magnitude of stress, may be determined by residual stress analyses.
The substrate of the PDC cutter may typically be made of cobalt-cemented tungsten carbide (WC), or other suitable cemented carbide material, such as tantalum carbide, titanium carbide, or the like. The cementing material, or binder, used in the cemented carbide substrate may be cobalt, nickel, iron, or alloys formed from combinations of those metals, or alloys of those metals in combination with other materials or elements. Experimental testing has shown that introduction of a selective gradation of materials in the substrate will produce suitable stress states in the carbide substrate and diamond table. For example, the use of varying qualities of grades or percentages of cobalt-cemented (hereinafter xe2x80x9cCo-cementedxe2x80x9d) carbides in the substrate produces very suitable states of compression in the diamond table and reduced residual tensile stress in the carbide substrate and provides increased strength in the cutter.
It has also been shown that a PDC cutter with suitably modified stress states in the diamond table and substrate may be formed by selectively manipulating the qualities of grades or percentages of binder content, carbide grain size or mixtures of binder or carbide alloys in the substrate. Thus, the specific properties of the cutter may be achieved through selectively dictating the metallurgical content of the substrate. Further, subjecting the PDC cutter of the present invention to an annealing step during the sintering process increases the hardness of the diamond table. Subjecting the formed (sintered) PDC cutter to a post-process stress relief anneal procedure provides a further means for selectively tailoring the stresses in the PDC cutter and significantly improves the hardness of the diamond table. Additionally, tailoring the thickness of the backing and/or subjecting the substrate to the disclosed annealing processes also provides selected suitable stress states in the diamond table and support.